Dimethyl ether (C2H6O, DME) is an ether with two methyl groups as organic residues. High-purity DME is widely used as fuel gas. Due to a cetane number of 55 to 60, dimethyl ether can also be used as a substitute for diesel fuel in a diesel engine. It is regarded as biofuel, if it is made from biomass, and on the long run it should replace liquefied gas. Therefore, DME has considerably gained importance in recent years.
The production of dimethyl ether can be effected by acid-catalyzed condensation of methanol with elimination of water:2 CH3OH→CH3—O—CH3+H2O
This process for producing dimethyl ether from methanol is described e.g. in “Ullmann's Encyclopedia of Chemistry, Sixth Edition, 1998 Electronic Release”. In this process, methanol vapor is passed over a reactor filled with dehydration catalyst, followed by a two-stage distillation. In the first distillation stage, dimethyl ether is recovered as top product, whereas the bottom product chiefly contains non-reacted methanol and product water. This bottom product is separated in the second distillation stage, wherein methanol is withdrawn as top product. The water obtained as bottom product is discarded or used in another process.
For practical reasons, a production of dimethyl ether on a technical scale often is provided directly subsequent to a process for recovering methanol.
For this purpose, WO 96/28408 A1 teaches a process for the production and recovery of dimethyl ether by dehydration of methanol. The energy consumption in the distillation should be reduced in that the bottom product obtained from the first distillation stage is passed over a stripper in which a rough separation of water and methanol is effected. The prepurified methanol stream then is fed into the methanol supply conduit and transferred into a distillation column provided upstream of the dimethyl reactor. As a result, the entire methanol stream fed into the reactor is processed and the purification of the bottom product after the DME reactor is shortened.
In a similar way, JP 2004/161673 A also tries to connect the separation of the product DME from unreacted methanol with a methanol recirculation. For this purpose, the entire crude methanol is purified in a distillation column and subsequently converted to dimethyl ether in a DME reactor. The bottom product obtained in the downstream distillation is recirculated into the crude methanol steam and along with the same into the distillative prepurification.